Operations With Vector Data I
HES 505 Fall 2023: Session 11
Matt Williamson
Today’s Plan
Image Source: USGS
Objectives
By the end of today, you should be able to:
Recognize the unary, binary, and n-ary transformers
Articulate common uses for unary and binary transformers
Use unary transformations to fix invalid geometries
Implement common binary transformers to align and combine data
Revisiting predicates and measures
Predicates: evaluate a logical statement asserting that a property is
TRUEMeasures: return a numeric value with units based on the units of the CRS
Unary, binary, and n-ary distinguish how many geometries each function accepts and returns
Transformations
- Transformations: create new geometries based on input geometries
Unary Transformations
| transformer | returns a geometry … |
|---|---|
centroid |
of type POINT with the geometry’s centroid |
buffer |
that is this larger (or smaller) than the input geometry, depending on the buffer size |
jitter |
that was moved in space a certain amount, using a bivariate uniform distribution |
wrap_dateline |
cut into pieces that do no longer cover the dateline |
boundary |
with the boundary of the input geometry |
convex_hull |
that forms the convex hull of the input geometry |
line_merge |
after merging connecting LINESTRING elements of a MULTILINESTRING into longer LINESTRINGs. |
make_valid |
that is valid |
node |
with added nodes to linear geometries at intersections without a node; only works on individual linear geometries |
point_on_surface |
with a (arbitrary) point on a surface |
polygonize |
of type polygon, created from lines that form a closed ring |
Unary Transformations (cont’d)
| transformer | returns a geometry … |
|---|---|
segmentize |
a (linear) geometry with nodes at a given density or minimal distance |
simplify |
simplified by removing vertices/nodes (lines or polygons) |
split |
that has been split with a splitting linestring |
transform |
transformed or convert to a new coordinate reference system (chapter @ref(cs)) |
triangulate |
with Delauney triangulated polygon(s) (figure @ref(fig:vor)) |
voronoi |
with the Voronoi tessellation of an input geometry (figure @ref(fig:vor)) |
zm |
with removed or added Z and/or M coordinates |
collection_extract |
with subgeometries from a GEOMETRYCOLLECTION of a particular type |
cast |
that is converted to another type |
+ |
that is shifted over a given vector |
* |
that is multiplied by a scalar or matrix |
Common uses of Unary Transformers
Creating valid geometries
Reprojecting your data
Combining or changing geometries
Binary Transformers
| function | returns | infix operator |
|---|---|---|
intersection |
the overlapping geometries for pair of geometries | & |
union |
the combination of the geometries; removes internal boundaries and duplicate points, nodes or line pieces | | |
difference |
the geometries of the first after removing the overlap with the second geometry | / |
sym_difference |
the combinations of the geometries after removing where they intersect; the negation (opposite) of intersection |
%/% |
crop |
crop an sf object to a specific rectangle |
Binary Transformers
Common Uses of Binary Transformers
Relating partially overlapping datasets to each other
Reducing the extent of vector objects
N-ary Transformers
Similar to Binary (except
st_crop)unioncan be applied to a set of geometries to return its geometrical unionintersectionanddifferencetake a single argument, but operate (sequentially) on all pairs, triples, quadruples, etc.